1. Field of the Invention
This invention relates to a control device for an inverter which operates the supply and reception of power to and from an AC system by interconnecting with the AC system, and more particularly relates to a control device for a system interconnection inverter which can continuously supply power to a load by the inverter alone even if the interconnection with the AC system is interrupted.
2. Description of the Related Art
System interconnection inverters are used for supplying power to loads from DC power sources, such as fuel cells, secondary battery cells and rectifiers. They are also used for receiving and supplying power between these DC power sources and AC systems.
FIG. 14 is a diagram showing a prior art example of a control device for this type of system interconnection inverter. The prior art control device is composed of a voltage source type self-commutated inverter 10 and an inverter control device 100. Voltage source type self-commutated inverter 10 is composed of an inverter main circuit 1 (described later), a DC capacitor 2 and a transformer 3. Inverter main circuit 1 has power conversion devices (controllable switching devices) GU, GV, GW, GX, GY and GZ and rectifying devices DU, DV, DW, DX, DY and DZ. Power conversion devices having self-turn-off ability, such as GTOs (gate turn-off thyristors), power transistors, IGBTs (insulated gate bipolar transistors) and SI (static induction type) thyristors may be used as power conversion devices GU, GV, GW, GX, GY and GZ. Self-commutated inverter 10 is interconnected to a 3-phase AC system 6 via an interconnection circuit breaker 5 and is also connected to a load 7.
Inverter control device 100 is composed of an active/reactive current reference generator 101, a phase detector 103, an active/reactive current detector 104, a current control circuit 105, a gate control circuit 106 and also Hall CTs 201,202 and 203.
Inverter main circuit 1 can control the 3-phase output voltage of inverter main circuit 1 by altering the conductive periods of power conversion devices GU, GV, GW, GX, GY and GZ. It also controls the current supplied to and received from AC system 6 via the impedance of transformer 3 by adjusting the phase and amplitude of the 3-phase output voltage of inverter main circuit 1 in response to the phase and amplitude of system voltages VR, VS and VT of AC system 6.
By means of this current control, inverter 10 supplies and receives active power to and from AG system 6 and also supplies reactive power to AC system 6 via interconnection circuit breaker 5 by converting the DC power of a DC power source 4 to active power or converting active power to DC power. Similarly, inverter 10 also supplies active power and reactive power to load 7.
Current control of inverter 10 is performed by inverter control device 100 as follows.
Phase detector 103 detects a phase .theta. of system voltages VR, VS and VT of 3-phase AC system 6 on the inverter 10 side.
Active/reactive current detector 104 detects the active current component and the reactive current component from inverter output AC currents iR, iS and iT which are detected by Hall CTs 201, 202 and 203, as respective active current detected value iq and reactive current detected value id.
Current control circuit 105 computes inverter output voltage references VRc, VSc and VTc, which determine the 3-phase output voltage of inverter main circuit 1, so that active current detected value iq and reactive current detected value id from active/reactive current detector 104 equals active current reference value iqc and reactive current reference value idc from active/reactive current reference generator 101. In the calculation of these inverter output voltage references VRc, VSc and VTc, the phase of the inverter output voltage for that of system voltages VR, VS and VT of AC system 6 are to be determined. Therefore, system voltage phase .theta. detected by phase detector 103 is used in the calculation.
Gate control circuit 106 compares inverter output voltage references VRc, VSc, and VTc with a triangular carrier wave signal produced within gate control circuit 106, and outputs gate signals which determine the conductive periods of power conversion devices GU, GV, GW, GX, GY and GZ composing inverter main circuit 1.
A detailed explanation of the operation of the system interconnection inverter and its control device shown in FIG. 14 have already been given in the reference A stated below. The detailed explanation is therefore omitted here.
Reference A: Shun-ichi Hirose et al. "Application of a digital instantaneous current control for static induction thyristor converters in the utility line". PCIM Proceedings, pp 343-349, Dec. 8, 1988 in Japan.
Also, the operation of gate control circuit 106 is given in the reference B stated below.
Reference B: Report of the Institute of Electrical Engineers of Japan, Specialist Committee on the Study of Semiconductor Power Conversion Methods, "Semiconductor power conversion circuits", pp 108-112, "PWM Inverter", published on Mar. 31, 1987 by the Institute of Electrical Engineers of Japan, Incorporated.
The prior art system interconnection inverter control device in FIG. 14 has the following problem. That is to say, when interconnection circuit breaker 5 opens due to the occurrence of a fault or the like in AC system 6, inverter 10 cannot execute the supply and reception of power with AC system 6 and, at the same time, the phase of the AC voltage of AC system 6 cannot be detected. Therefore, active current component iq and reactive current component id, which are detected from inverter output AC currents iR, iS and iT, cannot be outputted as active current reference value iqc and reactive current reference value idc from active/reactive current reference generator 101 as they should be. As a result, the output voltage and frequency of inverter 10 increase or decrease so that the desired power cannot be supplied to load 7. Therefore, the problem arises that the operation of inverter 10 has to be stopped.